1. Field of the Invention
The invention generally relates to electrochemical cells for use in powering implantable devices and in particular to an improved expanded screen current collector for use within an electrochemical cell.
2. Description of the Related Art
The surgical implantation of electronic devices in humans and animals has become a commonplace occurrence. Such devices are used for a wide range of purposes within the body. The most commonly known of such devices is the cardiac pacemaker. Other well-known implantable devices are employed for stimulating or sensing, or both, with respect to the brain, spinal cord, muscles, bones, nerves, glands, or other body organs or tissues.
Implantable devices are becoming more and more complex and commonly include sophisticated data processing hardware such as microprocessors, or related devices, ROM and RAM memories, LSI (Large Scale Integration) devices as well as other computer hardware. In many cases, information is transmitted to and from the implantable device to external monitoring equipment and such information may include device identification, biological data, parameters of present operation of the device (from previous settings), technical information concerning proper functioning of the device, patient and physician data, up-to-date programming for the device and verification of information transmitted to and from the device.
With more and more data being processed and available within the implantable device, there is a need to transmit more and more data from the implanted device to external devices for analysis, reprogramming of the implantable device, or for other purposes.
As a result of the increase sophistication of implantable devices and particularly due to the increased amount of data required to be transmitted from the device, the need to provide improved power sources for the implantable devices has increased greatly. There are, of course, limitations on the design of power cells for use in implantable devices, especially with regards to the size and shape thereof. Furthermore, the power supplies must be highly reliable and be capable of providing an adequate amount of current and voltage for an extended period of time.
One type of power supply for use in an implantable device is an electrochemical cell, particularly one employing lithium as an anode material. Typically, within such cells, a metal foil anode coated with lithium is provided in combination with a porous carbon cathode formed on a current collector, with a polymeric separator positioned between the anode and cathode. The electrode structure is mounted within a housing which is at least partially filled with a liquid electrolyte. It is desirable for the current collector to be configured to allow for a large amount of cathode material to be formed thereon while also achieving adequate electrical contact to all portions of the cathode material.
One commonly employed current collector configuration is an expanded screen wherein a flat, thin sheet of electrically conducting material, such as stainless steel, titanium or aluminum, is partially cut in numerous places along a direction extending between opposing side edges. Thereafter, the current collector material is pulled or stretched along a direction perpendicular to the cutting direction to expand the collector material and thereby yield a structure having a substantial surface area for receiving cathode material.
FIG. 1 illustrates a conventional expanded screen current collector 10. Arrows 12 and 14 indicate the direction along which the material is pulled or stretched to achieve the expanded configuration as illustrated. The current collector 10 includes numerous individual strings 16 extending generally from a bottom side edge 18 to a top side edge 20. Numerous openings 22 exist between adjacent strings 16. The openings 22 result from the expansion of the partial cuts formed in the current collector material. As can be seen from FIG. 1, the strings 16 project outwardly along the side edges 18 and 20 yielding sharp tines or projections 24. The sharp tines 24 often become bent during handling of the current collector.
An electrode structure employing the expanded screen current collector is illustrated in FIG. 2. The electrode structure 23 is formed by applying an appropriate amount of a cathode material 25 onto the expanded screen current collector 10 which is then mounted to one side of a polymeric separator 27. An anode 31 is mounted to an opposing side surface of the separator 27. Within FIG. 2, for clarity in illustrating the current collector 10, a portion of the electrode structure 23 is shown without the anode, separator and cathode materials. In actual electrodes, the current collector is completely covered by the cathode material and the anode and separator extend along an entire length of the current collector.
The sharp tines 24 of the current collector 10, if bent, can damage the polymeric separator 27 and can further cause a short circuit directly between the anode 31 and the cathode 25, thereby degrading and perhaps completely preventing performance of a resulting electrochemical cell.
FIG. 2 also illustrates an electrochemical cell housing 26 into which the electrode structure 23 is inserted. An arrow 33 illustrates a direction of insertion. During insertion, end portions of the electrode structure 23 are mounted within slots 28 and 29. (Only one exemplary set of slots are illustrated. An actual electrochemical cell will typically have many slots mounted in parallel.)
It has been found that damage caused by the sharp tines 24 is particularly severe within the vicinity of the slots 28 and 29.
To partially minimize the risk of damage, some expanded screen current collectors are formed as shown in FIG. 3 wherein only a central portion of an expanded screen current collector 11 is cut and expanded. Opposing end portions 30 and 32 are left unexpanded. The overall length of the expanded screen current collector, and of end portions 30 and 32, is selected to match the distance between opposing slots within a housing such as the one shown in FIG. 3. When the electrode structure is mounted within the housing, the end portions 30 and 32 are thereby properly received within the slots.
Hence, those portions of the expanded screen current collector which are mounted within the slots of the cell housing do not include any sharp tines and potential damage to the electrode structure in the vicinity of the slots is avoided. However, damage may still occur as a result of sharp tines formed along the expanded portions of the current collector. Moreover, because of the presence of the unexpanded end portions of the current collector, the resulting electrode structure can only be effectively employed within an electrochemical cell having a slot spacing matching the length of the electrode structure. Such a current collector can not be formed from an arbitrarily sized sheet, then cut to fit any of a number of predetermined slot spacings, as can be done for the expanded current collectors of the type illustrated in FIG. 1. Rather, once the expanded screen current collector of FIG. 3 is formed, it cannot be significantly cut in length to fit an arbitrary slot spacing but is useful only in an electrochemical cell having a slot spacing substantially matching the un-cut current collector length.
The foregoing problems which result from the presence of the sharp tines is particularly severe for electrochemical cells requiring very thin expanded screen current collectors. For thicker electrodes, such as electrodes having a thickness greater than 0.035 inches, any resulting sharp tines are usually sufficiently rigid to resist bending. Subsequent damage, if any, to the adjacent polymeric separator is thereby minimal. However, for certain electrochemical cells, an expanded screen current collector having such a thickness does not allow for a cell to be formed having an adequate geometrical surface area of current collector material. Such is a particular problem in small electrochemical cells for use in implantable devices which must be small in size. To provide a greater geometrical surface area and thereby improve the overall performance of such cells, it is desirable to employ a current collector having a thickness less than 0.035 inches, and in particular less than 0.025 inches. By employing thinner current collectors, a greater geometrical surface area is achieved without requiring a corresponding increase in the overall volume of the current collector.
However, for current collectors having a width less than 0.025 inches, particularly less than 0.020 inches, the aforementioned damage caused by the presence of the sharp tines is particularly problematic. With such a width, the resulting tines are considerably less rigid than in thicker current collectors, thus allowing the tines to be accidentally bent during fabrication or mounting in a manner which may damage the polymeric separator and perhaps cause a short circuit to the anode. It has been found that such problems are particularly severe when employing current collectors formed of stainless steel or titanium in part because such materials cannot be sufficiently annealed to conform to shape changes during assembly. The fabrication of cells incorporating thin current collectors formed of aluminum has been somewhat more successful, yet the overall performance of the resulting cells is generally unacceptable because aluminum does not provide the degree of current collection that can be achieved using titanium or stainless steel.
Many of the aforementioned problems occur for woven current collectors as well as for expanded screen current collectors.
It would be desirable to provide an improved method for fabricating expanded screen current collectors and woven current collectors for use in electrochemical cells, particularly those employed in implantable devices, wherein the aforementioned problems are substantially avoided. In particular, it would be desirable to provide a method for fabricating a current collector having a thickness less than 0.025 inches, yet formed of titanium or stainless steel, in a manner such that the current collector can be reliably employed within an electrochemical cell.